A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Assessment Program.

ToxicologyInformationBrief

How Much is a Part Per Million?

Publication Date: 9/93

INTRODUCTION

The health effects of any toxic substance are related to the
amount of exposure, also known as the dose. The greater the dose
the more severe the effects. Some chemicals can cause toxicity
at very low doses and so it is important to be able to understand
how these very small amounts are described. It is especially
important to understand how low doses compare to one another and
what they represent when compared to amounts of more familiar
substances.

Parts per million (ppm), parts per billion (ppb), and parts
per trillion (ppt), are the most commonly used terms to describe
very small amounts of contaminants in our environment. But what
do these terms represent? They are measures of concentration,
the amount of one material in a larger amount of another
material; for example, the weight of a toxic chemical in a
certain weight of food. They are expressed as concentrations
rather than total amounts so we can easily compare a variety of
different environmental situations. For example, scientists can
measure the concentration of a chemical in the Great Lakes by
looking at small samples. They do not have to measure the total
amount of chemicals or water in all of the lakes.

An example might help illustrate the part per ... idea. If
you divide a pie equally into 10 pieces, then each piece would be
a part per ten; for example, one-tenth of the total pie. If,
instead, you cut this pie into a million pieces, then each piece
would be very small and would represent a millionth of the total
pie or one part per million of the original pie. If you cut each
of these million minute pieces into a thousand little pieces,
then each of these new pieces would be one part per billion of
the original pie. To give you an idea of how little this would
be, a pinch of salt in ten tons of potato chips is also one part
(salt) per billion parts (chips).

In this example, the pieces of the pie were made up of the
same material as the whole. However, if there was a contaminant
in the pie at a level of one part per billion, one of these
invisible pieces of pie would be made up of the contaminant and
the other 999,999,999 pieces would be pure pie. Similarly, one
part per billion of an impurity in water represents a tiny
fraction of the total amount of water. One part per billion is
the equivalent of one drop of impurity in 500 barrels of water.

COMPARISONS AND CONVERSIONS

Sometimes, instead of using the part per ... terminology,
concentrations are reported in weight units; such as the weight
of the impurity compared to the weight of the total. The metric
system is the most convenient way to express this since metric
units go by steps of ten, hundred and thousand. For example, a
milligram is a thousandth of a gram and a gram is a thousandth of
a kilogram. Thus, a milligram is a thousandth of a thousandth,
or a millionth of a kilogram. A milligram is one part per
million of a kilogram thus, one part per million (ppm) is the
same as one milligram per kilogram. Just as part per million is
abbreviated as ppm, a milligram per kilogram has its own
abbreviation -- mg/kg. Using our abbreviations, one ppm equals
one mg/kg.

Kilograms and milligrams are units of weight so they don't
apply to volumes of liquids or gases. Instead of a kilogram, the
unit of liquid volume most commonly used is the liter. A liter
of water weighs one kilogram. If the contaminant is a solid, it
is measured in milligrams. Thus, one part per million of a solid
in a liquid can be written as a milligram per liter and
abbreviated mg/l.

These are the most common units that are encountered.
However, with the ability to detect even smaller amounts of
contaminants, the terms part per billion and part per trillion
are becoming more common. In the metric weight system, a
microgram is a thousandth of a milligram. Since a milligram is a
millionth of a kilogram, and the microgram is a thousand times
smaller, it is equivalent to a billionth of a kilogram.
Microgram is abbreviated ug. Thus, a part per billion of solid
measure is equal to a ug/kg. Similarly, a part per billion of a
solid in a liquid is equal to a ug/l.

Before going on to discuss a real example of how these
measurements are used, we can compare metric weight quantities to
the quantities we are most accustomed to using. A kilogram is
equal to about two pounds. Thus, a milligram is less than a
millionth of a pound. Looked at another way, it would take about
five thousand milligrams (5000 mg) to make up one teaspoonful of
a solid (such as salt). The unit of liquid volume, the liter, is
very close to a quart. Thus, a milligram per liter is about the
same as a milligram per quart.

THE CASE OF PCBs:

An Example

In order to appreciate how these quantities can be used in a
real situation, an example is in order. In this example, we use
the part per ... terminology to compare the relative importance
of PCBs in Great Lakes fish versus PCBs in Great Lakes drinking
water; that is, which source might contribute most to PCB
exposure of humans living in the Great Lakes states. The maximum
level of PCBs legally allowed in fish sold in interstate commerce
is 2 ppm (parts per million). Although there are no legally
established levels for PCBs in drinking water, measurements have
shown that the average PCB content of the Great Lakes drinking
water is about 4 ppt (parts per trillion).

Since a part per trillion is a million times less than one
part per million, the maximum allowable concentration of PCBs in
fish is about a million times higher than the level of PCBs in
drinking water. However, we generally consume a lot more water
than fish. At the extreme, people might eat as much as a pound
of fish a day or as little as one pound every 100 days (1/100
lb/day). On the other hand, people generally drink about 2
liters (equivalent to about 5 pounds) of water a day.

Thus, the consumption of water might range from about 5 to
500 times the consumption of fish. However, since there are a
million times more PCBs in a pound of fish compared to a pound of
water, fish can be a much greater source of PCBs than drinking
water. The total amount of PCBs consumed depends most on the
amount of fish eaten, how contaminated it is, and how it is
prepared. Thus, the best way to reduce human exposure to PCBs is
to reduce the levels in fish, reduce human consumption of fish
with the highest contaminant levels and prepare the consumed fish
in the most appropriate manner.

CONCLUSION

The ability to measure concentrations of chemicals in a
uniform manner provides a powerful tool for the comparison of
water quality from area to area, for the establishment of water
quality guidelines or a comparison of doses of chemicals as are
commonly found throughout the Pesticide Information Profiles.
The use of the metric system provides an easy way to utilize both
liquid and solid measurements.